Conventionally, dye inks were in the main stream as inkjet inks because of their excellent coloring ability and high reliability. However, as the dye inks have disadvantages such as poor water resistance and poor light resistance, pigment inks have been attracted more attentions in these days. In the pigment ink, a polymer compound is used for stably dispersing a pigment in water, or for fixing the pigment onto recording media after depositing the ink thereon.
Meanwhile, inkjet recording devices have their trends that diameters of nozzles serving as an ink ejecting unit have been getting smaller for achieving high image quality and high speed printing. However, the pigment ink containing the polymer compound tends to cause aggregations of its solids content as the moisture evaporates, and it is difficult to secure jetting stability of the ink when the ink is used in the inkjet recording device having the nozzle of a small diameter. To solve this problem, various attempts have been made for improving the jetting stability of the pigment ink containing the polymer compound.
For example, there is disclosed in PTL 1 that an ink has a viscosity change of 10 times or less when the ink is concentrated to have a doubled concentration. In accordance with this proposal, the disclosed ink has excellent jetting stability, and suppresses spreading of the ink because of the aggregation of the pigment when it is used to print on an inkjet printing paper, as well as preventing generating white missing spots in a resulting image. However, the ink specifically disclosed as the ink having the viscosity change of 10 times or less when the ink is concentrated to have a doubled concentration has a low pigment concentration, and low viscosity. Even such the ink is used for printing on plain paper, it is difficult to achieve to provide high quality images. Moreover, there is no teaching, PTL 1, about a method for preventing increase in the viscosity of the ink having a high concentration of the pigment when the moisture is evaporated.
Moreover, PTL 2 discloses an ink which has a viscosity increase rate (mPa·s/%) of 5.0 or less with the moisture evaporation when the moisture evaporation amount is up to 30% by mass relative to the total weight of the ink, and the viscosity increase rate of more than 50 when the moisture evaporation amount is 30% by mass to 45% by mass. It is disclosed that with this proposed ink high quality images are provided by high speed printing as the ink dramatically increases its viscosity when it is deposited on plain paper. However, the proposed ink has a problem that the jetting stability of the ink decreases if the ink is dried in a nozzle of an inkjet recording device.
Furthermore, PTL 3 discloses an ink containing hydroxyethylformamide for securing low viscosity and desirable jetting stability of the ink. However, hydroxyethylformamide contained in the proposed ink generates formamide when decomposed, and there are problems in safety (e.g. mutagenesis and genotoxicity) to human bodies.
As described above, it has been considered that it is necessary to use an ink the viscosity of which dramatically increases along with the evaporation of the moisture for forming high quality images on plain paper by high speed printing, but it is the current situation that it is difficult to secure the reliability of such the ink, and is difficult to utilize the ink. Especially in terms of the jetting stability, aggregations and drying of the pigment adjacent to the nozzle opening is a big problem. To solve this problem, various ink materials have been developed, but it is the current situation that it is still difficult to achieve both low viscosity of the ink and reduction of a solvent evaporation of the ink (drying adjacent to the nozzle opening).